Batch and flow microcalorimeters continue to be improved upon for the purpose of reliably measuring total reaction heats of less than one millicalorie using non more than 400 microliters of each reagent. Various artifacts have so far limited the usefulness of the flow system, but these are being systematically eliminated and a system should be operational soon. DNA reactions with clipping enzyme are presently under study as a reference system for the more interesting study of the assembly reactions of supercoiled DNA. The batch calorimeter is operational for this work but only one experiment can be done every two to three hours and this long wait time is very deliterious to the enzyme. With the flow system a reaction can be run every 15 minutes, thus making this type of experiment a very useful method of following the biochemical reaction.